This issue was raised by Dieter De Paepe through the mailing list:
The Related Service property is mapped to dct:relation, which has a pretty vague definition: "A related resource...". As this might overlap with other uses of dct:relation, I suggest subclassing this property.
Same remark as previous for use of spatial, which is defined as "Spatial characteristics of the resource."
Unless there is a specific need from the WG, we would propose to leave both properties as they are since we want to reuse existing specifications as much as possible.